To the Instant Pot inner pot, add the garlic, cauliflower florets, cashews, cannellini beans, vegetable broth, thyme, rosemary, bay leaf, nutritional yeast, 1 ¾ teaspoons kosher salt, and black pepper to taste. Stir to combine.
Secure the Instant Pot lid and set the Pressure Release to Sealing. Select the Pressure Cook setting and set to high pressure for 7 minutes.
Once the 7-minute timer has completed and beeps, perform a Quick Pressure Release by switching the Pressure Release knob from Sealing to Venting.
Open the pot and remove any whole herb sprigs. Use an immersion blender to puree the soup until thick and creamy, taking care to blend all the cashews. Alternatively, if you want a super silky texture, transfer the soup to a stand blender and blend until creamy and smooth (replace the center cap from the blender lid and cover with a dish towel to allow steam to escape).
Taste for salt and pepper and adjust accordingly (I usually add a bit more salt at this stage).
Transfer the soup to bowls, and before serving, zest some lemon on top, add a scattering of red pepper flakes, and drizzle with a bit of extra virgin olive oil. Serve with bread, if desired.